The invention relates to a system for the treatment of ashes or residues from the combustion of carbonaceous fuels, such as coal. The invention discloses both methods and apparatus to control various physical and chemical characteristics of combustion ash as they relate to cold bonding processes, and as they relate to the cured consolidated materials which result from these processes. Specifically, this invention relates to cured consolidated combustion ash materials which have been standardized for use as normal weight and light weight aggregate for use in structural and landfill applications.
The combustion of carbonaceous fuels for the production of electricity or process steam by the utility and industrial sectors is a major generator of combustion ash. Combustion technologies such as fluidized bed combustion (FBC) and pressurized fluid bed combustion (PFBC) are widely implemented. Apprehension about pollution from the smoke stack industries and utilities has led to the implementation of clean coal technologies that addresses flue gas contaminants, not only for particulate, but also gaseous emission, such as sulfur oxides. These flue gas desulfurization (FGD) technologies are widespread and cover a range of techniques including wet scrubbers or wet FGD; dry scrubber FGD (i.e., spray driers); sorbent injection technologies; and fluidized bed combustion (FBC) technologies each of which produce a particular type of ash as a by-product.
The development of re-use technologies for each of these ashes, as well as those that result from the combustion of carbonaceous fuels without FGD technologies, has been slow. The obstacles are both technical, as well as regulatory and legislative. One of the prominent technical issues is the inability to produce ash-based products which have certain required engineering properties or meet particular standards in the industry.
There are a number of ashes, for example FBC ashes, which contain large amounts of free lime and other oxides, such as those of magnesium, iron, sodium and potassium, which heat and expand upon combination with water. These exothermic and expansive hydration reactions can be detrimental to the dimensional stability of the conditioned, consolidated, or compacted ash in either landfill disposal or in other re-use applications. FBC and FGD combustion ashes are examples of ashes which tend to exhibit expansion sufficient to limit their options for use and often cause difficulty in the construction of stable landfills as discussed in xe2x80x9cAsh Management Options For AFBCxe2x80x9d, A. E. Bland and C. E. Jones which is hereby incorporated by reference. An approach to dealing with the problem of expansion in oxide rich combustion ash is disclosed by U.S. Pat. Nos. 5,364,572; 5,100,473; 4,250,134; 4,344,796 and by xe2x80x9cA New Approach To Hydration Of FBC Residuesxe2x80x9d by J. Blodin and E. J. Anthony. Each teaches that all the free lime or other oxides must be initially slaked or nearly completely converted to a non-expansive hydrate prior to further processing steps. However, several problems are associated with using sufficient water to slake or nearly completely convert oxides to the corresponding hydrates which relate to handling problems and reduced early strength development in the cured consolidated combustion ash materials.
Another primary concern related to the production of construction related materials from certain lime and sulfate containing ashes, such as FBC ashes, is the subsequent formation of the minerals such as calcium sulfo-aluminate hydrate (ettringite), calcium sulfate di-hydrate (gypsum), calcium silicate hydrates and calcium aluminate hydrates as disclosed in xe2x80x9cEffect of Curing Conditions on the Geotechnical and Geochemical Properties of CFBC Ashesxe2x80x9d, Proceedings of the 15th International Conference on Fluidized Bed Combustion, A. Bland, 1999 which is hereby incorporated by reference. The slow formation of these compounds has been linked to the observed expansion in FBC and other ashes, poor strength development in consolidated combustion ash materials and with the disintegration of cured consolidated materials. As cured consolidated combustion ash materials age, the formation of such minerals may continue and subsequently a portion of the pore volume within the material. The deposition of these minerals in the pores of the cured consolidated combustion ash material, left unchecked, may ultimately create enough force to crack the cured material adjacent to the pore. These micro-cracks may lead to a substantial loss of strength and abrasion resistance in the cured consolidated combustion ash material. A number of researchers have shown the benefit of soluble silicate addition to ashes containing free lime, such as FBC ashes. For example, as disclosed by U.S. Pat. Nos. 5,002,611 and 5,152,837 which focus on the addition of other ashes having soluble silicates to FBC ash. The soluble silicates in the ash react with the free lime in the FBC ash and form calcium silicates preferentially to the compound ettringite. This approach, however, has at least two problems associated with it. First, it is applicable to only certain ashes, and secondly it is costly because it requires additional steps related to procuring fly ash with a suitable amount of soluble silicate and the additional steps of processing the fly ash with the FBC ash.
Another significant problem in the field is the increasing use of self cementing combustion ashes, such as FBC ash, which contain large amounts of free lime and other oxides to replace the use of costly cement and lime additives in external cold bonding processes as disclosed by U.S. Pat. Nos. 4,624,711; 5,512,837; and 5,766,338 to increase the strength of the cured consolidation combustion ash product. As the use of self cementing ashes in other processes has increased, there has been an increased and unresolved need for effective and economic processes for controlling the expansion in conditioned and consolidated combustion ash due to hydrate or mineral formation.
Another basic problem which exists with regard to processing FBC and other dry ash relates to the existing practice of combining the dry combustion ash, the water and other additives. Cured consolidation materials which result from existing apparatus and methods which combine dry combustion ash, water and other additives at low energy may not develop optimum strength, may have high permeability, or may also have increased amounts of expansion. The amount of energy used to combine these components can be quantified with reference to xe2x80x9cStandard Method For Mechanical Mixing Of Hydraulic Cement Pastes and Mortars of Plastic Consistencyxe2x80x9d, ASTM C305-82 which is hereby incorporated by reference. In actual practice, this standard has been used to quantify the amount of energy with which wet flue gas desulfurization sludge (FGD sludge) is processed as disclosed in U.S. Pat. No. 4,613,374. Blending FGD sludge in a pug mill for about 20 to about 40 seconds is a common commercial processing practice and has been equated to about eight seconds of mixing in a Hobart N-50 mixer set at speed level 1. U.S. Pat. Nos. 4,613,374 and 5,211,750 disclose that the manner of can be beneficial with regard to processing FGD sludge and perhaps other materials that have thixotropic properties. Materials, such as FBC and other dry ashes, are not thixotropic and yet an unexpected relationship exists with regard to how dry ash is combined with water and the enhancement of various characteristics of cured consolidated combustion ash materials.
Another problem related to combining combustion ash with water and other additives is the ability to disperse the water throughout the combustion ash solids evenly at low water to solids ratios. However, water to combustion ash solids ratios which are above about 0.30 may not achieve the level of strength which may be achieved using identical types of combustion ash at water to combustion ash solids ratios less than about 0.30.
From the commercial manufacturing perspective there remain several significant problems to resolve.
First, with regard to the use of sintering processes, such as those disclosed by U.S. Pat. Nos. 3,765,920; 4,772,330; 5,342,442; and 5,669,969, the processes are becoming increasingly less economical because of the high costs of energy required to produce the products at temperatures of between about 1650 to about 2190 degrees Fahrenheit, and because of the high maintenance cost of the sintering and mechanical handling equipment. Also, the sintering processes do not appear promising because certain ashes that contain sulfate and sulfide from FBC and FGD technologies result in unacceptable SOx emissions during the sintering process.
Secondly, with respect to cold bond processes, many products prepared from combustion ash by such processes do not meet existing standards for use as normal weight aggregate which limits the price the market will pay for the materials and limits the markets in which the materials may be introduced. Normal weight aggregates must meet or exceed standards for road base and concrete aggregate use as set forth by ASTM C-33 which is hereby incorporated by reference. Adjusting the strength of and limiting the linear expansion of cured consolidated combustion ash material produced by existing cold bonding processes, such as those disclosed by U.S. Pat. Nos. 4,624,711; 5,152,837; 5,002,611 and as practiced by the Aardalite and Agglite process, requires the use of additives or additional processing steps which maybe prohibitively expensive. Additionally, cold bond processes have had limited success when applied to some types of combustion ash such as Class FBC, dry FGD, sorbet injection and others.
Specifically, with regard to the lightweight aggregate market, the use of specialty chemicals and additives may be justified because lightweight aggregate commands higher market prices relative to normal weight aggregate. However, even where lightweight additives achieve the required reduction in density there may be an associated decrease in the strength and durability of the lightweight material making it unacceptable for sale in the lightweight aggregate market. There is a need for a cold bonding process which produces a cured consolidated combustion ash material with sufficient strength such that a variety of additives may be introduced to reduce the density of the material and still meet the other applicable standards for lightweight material.
The invention discloses basic ideas and concepts which address each of the above mentioned problems relating to cured consolidation materials from cold bonding processes. As such, the invention provides apparatus and methods for the processing, consolidation and curing of combustion ash to form novel materials, as well as, for the improvement of various characteristics relating to cured combustion ash materials processed by existing technology.
Accordingly, it is the broad object of the present invention to provide a system for the production of cured consolidated combustion ash materials from cold bonding processes having novel or enhanced characteristics. One specific goal in this respect is to provide both methods and apparatus for a combustion ash cold bonding process to produce standardized normal weight aggregate for use in road base; for use in concrete having structural, masonry and insulation applications, and for use in light weight aggregate applications.
A second broad objective of the invention is to provide technology which may improve previously disclosed or presently used processes for cold bonding combustion ash. Such technology, may improve the products of existing technologies so as to be more readily accepted in existing markets, or may allow the improved products to be introduced into new markets, or simply make the existing technologies more economical.
A specific objective of the invention is to control the amount of expansion of combustion ashes which contain high levels of oxide when the combustion ash is combined with water. Controlling the molar volume expansion of oxides may decrease the linear expansion and improve the unconfined compressive strength of cured consolidated combustion ash materials. For certain applications, the hydrated combustion ash material may be intentionally designed to expand to fill a specific volume.
Another specific objective of the invention is to control the potential for the formation of minerals, such as enttringite and gypsum, in cured consolidated combustion ash materials. Controlling the potential for formation of such minerals and in turn the amount of deposition of such minerals in the pore volume of consolidated or cured combustion ash materials may allow manufacturers to predetermine and select certain attributes of the cured consolidated combustion ash material to meet certain specifications, or the requirements of various markets.
Another specific objective of the invention is to assess and control the manner of combining various types combustion ash with water and other additives. By controlling the manner of combining the combustion ash with water, various characteristics of the cured consolidated combustion ash product may be adjusted. A significant goal in this respect is to broaden the achievable range for some characteristics. Specifically this may allow for cured consolidated combustion ash materials having increased density and unconfined compressive strength, as well as decreased linear expansion. The manner of combining the combustion ash with the water may also result in a density, such as that achievable using a ASTM D-1557 compactive effort, using a lower standard of compactive effort, such as a ASTM D-698 compactive effort. Since density is related to characteristics such as strength development, permeability, LA abrasion resistance, soundness, and expansion, a simple and economic method to increase density with reduced effort would be highly valuable tool. A related goal is to reduce the water-combustion ash solids ratios of the combined material. Another related goal is to decrease the need for additives which are presently used to impart increased strength to cured consolidated combustion ash materials produced by existing processes. Similarly, another goal is to allow for the use of a wider variety of combustion ashes in the production normal weight and lightweight aggregates.
Another specific objective of the invention is to increase the strength of cured consolidated combustion ash material so as to compensate for the decrease in strength attributed to the incorporation of light weight filler materials. A specific goal with respect to increasing the strength is to identify additives which have been shown to be beneficial in enhancing strength development of combustion ashxe2x80x94water combination materials. Another specific goal is to identify novel fillers which produce bubbles within the combination material prior to the time the combination material sets.
Yet another objective of the invention is to disclose apparatus and methods which allow for the use of or improvement in the processing of a wide variety of combustion ashes, examples include, fly ash which resulting from the combustion of fossil fuels which is entrained in flue gases and is then collected; bottom ash resulting from the combustion of fossil fuels that does not become entrained in the flue gases and is removed from the bottom of the combustor; bed ash resulting from the combustion of FBC, CFBC, or PFBC combustion of fossil fuel; Class C ash resulting from the combustion of low ranked coals, such as lignites and subbituminous coals, which meet the specifications of ASTM C-618 which is hereby incorporated by reference; off specification Class C combustion ash which result from the combustion of low ranked coals such as lignites and subbituminous coals, but which do not meet all the specifications of ASTM C-618; Class F ash resulting from the combustion of bituminous and anthracite coal which meets the specifications of ASTM 618; off specification Class F ash resulting from the combustion of anthracite and bituminous coals but which do not meet the specifications of ASTM 618; spray dryer ash produced from the spray driers used to clean the flue gases from the combustion of fossil fuels; sorbent injection ashes produced from the injection of a sorbent to capture gaseous sulfur and incinerator ash from the combustion of municipal waste or other ash types as they are identified or are developed. Still another objective of the invention is to make use of a variety of waste waters such as coal pile runoff produced by rain percolating through or running off coal piles which are required to be treated before discharge; cooling tower blowdown produced at power plants as a result of being associated with cooling towers and which must be treated prior to discharge; paper mill liquors or effluent produced in association with paper mills and which require treatment prior to discharge. Seawater and brackish water (seawater) associated with coastal influx of seawater may also be used.
Another objective of the invention to provide set retarding chemicals which retard the rate at which consolidated combustion ash material sets. These additives are critical to processing some types of self cementing combustion ash, and with respect to others allows certain techniques to be accomplished prior to the time the consolidated combustion ash materials set.
Specifically with regard to consolidated combustion ash material to be introduced into the market for normal weight aggregate, it is an objective of the invention to meet or exceed American Society for Testing Materials (ASTM) and American Association of State Highway Transportation Officials (AASHTO) specifications, which are hereby incorporated by reference.
ASTM and AASHTO specification relate to normal weight aggregate to be used in concrete (ASTM C-33 and AASHTO M-80 and M6); aggregate specifications for use in masonry grout (ASTM C-404); aggregate use in masonry mortar (ASTM C-144 and AASHTO M-45); aggregate specifications for use in highway construction, road and bridge construction and highway and airport base and subbase applications (ASTM D-448, ASTM D-2940 and AASHTO M 43); fine aggregate for bituminous paving mixtures (ASTM D-1073 and AASHTO M 29); specifications for mineral filler for bituminous paving materials (ASTM D-242 and AASHTO M 42); crushed aggregate for Macadam pavements (ASTM D-693); crushed stone, crushed slag, and gravel for single and multiple bituminous surface treatments (ASTM D-1139); material for soil aggregate subbase, base and surface courses (ASTM D-1241); and materials for aggregate and soil-aggregate subbase, base and surface courses (and AASHTO M 147).
With respect to lightweight aggregate for use in structural concrete, masonry units and insulating concrete, an objective of this invention is to meet or exceed ASTM and AASHTO specifications related to lightweight aggregate use in structural concrete (ASTM C-330 and AASHTO M-195), concrete masonry units (ASTM C-331), and in insulating concrete (ASTM C-332).
With respect to structural fill and landfill use, an objective of this invention is to meet or exceed AASHTO specifications for materials for embankments and subgrades (AASHTO M 57) and ASTM standards for ash use in structural fill applications. An objective of this invention is to produce a stable low expansion landfill with low permeability and adequate strength development.
With respect to producing expandable grouts, an objective of this invention is to meet or exceed any ASTM specifications for use of the material for filling mine voids (provisional standards being developed).